Rota White-eye (Zosterops rotensis)

Compiled by: Fred Amidon, USFWS, Honolulu, HI Order: Passeriformes Family: Zosteropidae Local or Chamorro Name: Nosa Luta SPECIES OVERVIEW Description: The Rota White-eye or nosa is a sexually monomorphic forest bird. The name white-eye is derived from the white ring of feathers around each eye. Their plumage is tinged with yellow, and their bill, legs, and feet are yellow-orange (Pratt et al. 1987).

Distribution and Status: Endemic to the island of Rota, Commonwealth of the Northern Mariana Islands. The Rota white-eye was originally classified as one of three subspecies of Bridled White-eye (Zosterops conspicillatus) found in the Mariana Islands. Stresemann (1931) described subspecies on the islands of Guam (Z. c. conspicllatus); Saipan, Tinian, and Aguiguan (Z. c. saypani); and Rota (Z. c. rotensis; hereby referred to as Guam Bridled, Saipan Bridled and Rota white-eyes respectively). However, based on recent genetic work (Slikas et al. 2000) and observed differences in plumage, vocalizations, and behavior (Pratt et al. 1987, Collar et al. 1994), the Rota white-eye is treated here as a full species. This species is currently restricted to forested areas above 150 meters elevation in the Sabana Region of Rota. The total population consists of approximately 1,000 birds (Fancy and Snetsinger 2001) and is believed to have declined from approximately 10,000 birds in 1982 (Engbring et al. 1986).

Habitat: Little research has been done on the habitat requirements of the Rota White-eye. Therefore, it is not possible to present a detailed analysis of their habitat needs. General habitat associations, however, can be inferred from survey data and the results of research by Amidon (2000), at least for the Rota white-eye population since 1982. Since the first island-wide forest bird survey in 1982, Rota white-eyes have been recorded primarily above 150 m elevation in the Sabana region of Rota (Engbring et al. 1986, Engbring 1987, Engbring 1989, Amidon 2000, Fancy and Snetsinger 2001, USFWS unpubl. data). Sightings of Rota white-eyes have been recorded in limestone forest, introduced Acacia confusa forest, introduced Leucaena leucocephala forest, and secondary vegetation (Craig and Taisacan 1994, Amidon 2000, Fancy and Snetsinger 2001, Amidon unpubl. data). However, the majority of the Rota white-eye sightings have been recorded in limestone forest. For example, of the survey stations where Rota white-eyes were detected in 1982 (n = 44; Engbring et al. 1986) and 1987 (n = 24; Engbring 1987), 89 percent (n = 39) of the stations in 1982 and 79 percent (n = 19) of the stations in 1987 were classified as limestone forest within 50 meters (160 feet) of the survey station by Falanruw et al. (1989). Of the remaining stations with Rota white-eye detections in 1982, 8 percent (n = 4) were in areas with mixed vegetation types that included some limestone forest and 2 percent (n = 1) were in forest other habitat types (e.g., Cocos nucifera (coconut palm) plantation and secondary vegetation). Of the remaining stations with Rota white-eye detections in 1987, 21 percent (n = 5) were in areas with mixed vegetation types that included some limestone forest. Further, of the stations with Rota white-eye detections in limestone forest in 1982 (n = 39) and 1987 (n = 19), over 60 percent of the areas were dominated by mature limestone forest with large diameter trees (> 30 cm dbh), high density, and over 70 percent canopy cover (Falanruw et al. 1989). A similar pattern was also observed for the 1996 survey by Fancy and Snetsinger (2001) were 73 percent of the Rota white-eye locations (n = 62) were recorded in areas classified as mature limestone forest by Falanruw et al. (1989). In 1998 and 1999, Rota white-eye habitat relationships were assessed within their current range and across the Sabana region as part of a two year study by Amidon (2000). Forested areas with high densities of Rota white-eyes (≥ 2 birds per hectare) had higher volumes of epiphytic plants, such as Asplenium nidus and Davallia solida, and were primarily composed of Elaeocarpus joga, Hernandia labyrinthica, Merrilliodendron megacarpum, Pandanus tectorius, and Premna obtusifolia trees. Other tree species that were regularly recorded in Rota white-eye high density areas include Aglaia mariannensis, Artocarpus atilis, Ficus prolixa, Ficus tinctoria, Guettarda speciosa, Macaranga thompsonii, and Pisonia umbellifera. Within the Rota white- eye’s range, white-eyes were found to be more abundant in areas with higher densities of Elaeocarpus joga and high foliage volume. Rota white-eye abundance was also found to have a positive relationship with the abundance of Merrilliodendron megacarpum. Across the Sabana, Rota white-eyes were found to be more abundant in areas with high densities of Hernandia labyrinthica and where the groundcover species Elatostema and Procris spp. were present. Food and Feed Habits: Very little is known about the food habitats of Rota white-eyes. They are believed to feed primarily on insects, however, they have been observed foraging on the fruits of Pipturus argenteus and Macaranga thompsonii trees and probing the flowers, presumably to feed on nectar, of Elaeocarpus joga, Hernandia labyrinthica, Macaranga thompsonii, Persea americana, Premna obtusifolia, and Eugenia thompsonii trees (F. Amidon, unpubl. data). Rota white-eyes forage primarily by gleaning insects from leaves and branches of trees (Craig and Taisacan 1994, Amidon 2000). However, they have been observed sallying for insects, probing flowers for insects or pollen, and searching for food in epiphytes and moss (Amidon 2000; F. Amidon, unpubl. data). Rota white-eyes typically forage in the outer layer of canopy trees on perches less than 1.0 centimeter diameter (Craig and Taisacan 1994, Amidon 2000). Of 97 Rota white-eye foraging observations, the majority were reported in Elaeocarpus joga (34 percent), Hernandia labyrinthica (13 percent), Macaranga thompsonii (10 percent), Merrilliodendron megacarpum (9 percent), and Premna obtusifolia (9 percent; Amidon 2000; F. Amidon, unpubl. data). However, Rota white-eyes were also recorded foraging in Pipturus argenteus, Persea americana, Guettarda speciosa (panao), Ficus tinctoria (hodda), Acacia confusa, Aglaia mariannensis (mapunyao), Eugenia thompsonii, Ficus prolixa (nunu), Tarenna sambucina (sumac-lada), and Tristiropsis obtusangula (faniok) trees (F. Amidon, unpubl. data). Behavior: Rota white-eyes have been observed making several vocalizations. The most commonly observed vocalization is a call that Pratt et al. (1987) described as “a low-pitched tsheip.” They have also been observed giving a scolding alarm call, often in response to collared kingfishers (Todiramphus chloris), and have been observed singing in the upper branches of canopy trees (Amidon 2000). Like many of the white-eyes in the family Zosteropidae, Rota white-eyes are gregarious and are often observed in small groups. These groups typically consist of 2 to 3 birds (53 percent, n = 154) and sometimes included rufous fantails (Rhipidura rufifrons; Amidon 2000). Based on observations of frequent food begging and mutual preening or allopreening, Craig and Taisacan (1994) and Amidon (2000) believed that these small groups were composed of related individuals. Larger groups of 4 to 5 birds are observed occasionally (18 percent, n = 154) and groups of up to 14 birds are observed very rarely (1 percent, n = 154; Amidon 2000). In contrast, Craig (1989) typically observed Saipan bridled white-eyes in flocks of 10 to 40 individuals. Historically, Rota white-eye group sizes were reported to be larger and available evidence indicates that group sizes have decreased as the population declined (Craig and Taisacan 1994, Fancy and Snetsinger 2001). Breeding: Observations of breeding activity indicate that Rota white-eyes breed from at least December to August (Lusk and Taisacan 1997; Amidon et al. 2004). However, the species may breed year-round, as was reported for the Guam bridled white-eye (Marshall 1949, Jenkins 1983), because nesting has been observed in both the wet and dry seasons. Nesting: Rota white-eye nests are cuplike and typically suspended between branches and branchlets or leaf petioles (Yamashina 1932, Lusk and Taisacan 1997, Amidon et al. 2004). However, one nest was observed suspended from Davallia solida ferns below the branch of a tree (Amidon et al. 2004). Nests appeared to be composed of rootlets, woven grass or Pandanus spp. fibers, spider webs, light green moss, and a yellow, cottony material (Yamashina 1932, Lusk and Taisacan 1997, Amidon et al. 2004). The inner cup appeared to be of woven grass or Pandanus spp. fibers. Nest dimensions have been recorded for 6 nests (Yamashina 1932, Lusk and Taisacan 1997, Amidon et al. 2004). Mean nest height was 43.2 mm (range, 36.0-50.0 mm) and mean cup depth was 28.5 mm (range, 25.0-30.0 mm). Mean cup diameter was 43.9 mm (range, 44.6-50.0 mm) and mean nest diameter was 62.6 mm (range, 57.7-70.0 mm). Rota white-eyes have been reported nesting in the native tree species Hernandia labyrinthica (n = 9), Merrilliodendron megacarpum (n = 27), and Elaeocarpus joga (n = 7), and the introduced tree species Acacia confusa (n = 3) between approximately 150 and 460 meters elevation (Lusk and Taisacan 1997; Amidon et al. 2004; E. Taisacan, pers. comm., 2005; F. Amidon, unpubl. data). Pratt (1985) also reported finding a nest in a Hernandia spp. (presumably H. labyrinthica based on the location where the nest was found). The mean distance of 23 nests from the ground was 7.7 m (range, 2.5-12.8 m). The mean height of 18 nest trees was 10.1 m (range, 3.3-14.6 m) and the mean diameter at breast height for 19 nest trees was 28.2 cm (range, 2.3-60.2 cm). Mean distances of 19 nests from the boles of the nest tree was 3.0 m (range, 0.8-6.7 m). Eggs, Incubation, Hatching, Growth, and Development: Both male and female Rota bridled white-eyes incubate, brood, and feed nestlings (Amidon et al. 2004). Eggs are light blue and clutch sizes of one to two eggs have been observed (Yamashina 1932, Amidon et al. 2004), though clutch sizes of three eggs are possible based on observed clutch sizes for bridled white- eyes on Guam, Tinian, and Saipan (Hartert 1898, Yamashina 1932, Sachtleben 2005). Observations of seven active nests indicate that incubation and nestling periods appeared to be at least 10 and as long as 12 days for Rota white-eyes (Amidon et al. 2004). The post-fledging parental attendance period is unknown, but observations of one banded nestling indicate it is at least 8 days (Amidon et al. 2004). Threats: Among the factors that have been hypothesized to threaten the Rota white-eye are: habitat loss or degradation; predation by introduced rats and black drongos, and other predators; the accidental introduction of new predators, like brown treesnakes; avian disease; and random catastrophic events, like typhoons, which may affect the core range of the species and lead to its extinction. Of these factors, habitat loss and degradation and predation by introduction species are currently believed to be the primary factors in the population decline and core range restriction of the Rota white-eye. Research Needed: Foraging and nesting habitat requirements and sources of mortality. Conservation Recommendations: Protection of remaining native forest utilized by species is considered essential along with restoration of degraded habitats (specifically areas converted to Pandanus spp. thickets). Management of primary sources of nest and adult mortality should also be implemented once those sources are identified through the research above. Establishing additional populations of Rota white-eyes outside their current range (in captivity and/or the wild) should be considered to help protect the species from catastrophic events.